The present invention relates to an encoding technology of encoding image data to compress an amount of the data, and more particularly, to an encoding method and apparatus in which a quantization level is varied according to an energy characteristic of a spatial frequency domain.
Recently, a system for transmitting and receiving an image signal and an audio signal chiefly uses a method for converting an analog image and audio signal into a digital image and audio signal and encoding the converted digital signal to transmit the encoded signal and store the converted signal in a storage device. The system further uses a method for receiving the transmitted signal or reading out the stored signal to reproduce the received signal by decoding the received signal or the read-out, signal, and converting the decoded signal into an analog signal. Such an encoding and decoding system requires technology for compressing a larger amount of transmitted data to maximize a transmission efficiency of the data.
General methods for encoding an image signal which exist are, for example, a transformation encoding method, a differential pulse code modulation method, a vector quantization method and a variable length coding method. The encoding methods remove redundancy data included in digital image signals for use in compression of a total amount of the data.
To perform such an encoding method, a picture is divided into blocks, each of which has a predetermined magnitude, and image data is transformed into a transformation coefficient of a frequency domain by way of predetermined transformation with respect to each block or a difference signal between the blocks. A discrete cosine transform, a Walsh-Hadamard transform, a discrete cosine transform or a discrete sine transform, for example, can be used as data transformation for each of the respective blocks of data. Technologies for properly encoding such transformation coefficients according to the data characteristics, storing or transmitting the encoded signal, and decoding the encoded signal for reproduction have been recognized as very important technologies in a high definition TV (HDTV), a high definition VTR (HD-VTR), a digital camcorder, multimedia equipment, a video phone, etc.
Meanwhile, when an image signal is compressed using a variable length coding and decoding apparatus, a buffer is used for invariably maintaining a transfer bit rate of the encoded and transmitted signal. A quantization level is controlled according to a fullness of the buffer to avoid an overflow or an underflow of the buffer. Accordingly, an amount of the data which is input to the buffer is controlled. A conventional quantization level is chiefly controlled in units of a slice according to the data storage state in the buffer. In such a quantization level determination method, the quantization level is properly controlled according to the buffer fullness when the transfer bit rate of the buffer is determined. Thus, an amount of the bits which are input to the buffer could be exactly controlled. However, there exists a problem that an image characteristic is not considered at all.
Accordingly, many of the systems varied the quantization level in units of a macroblock so that the image characteristic could be reflected. Such a conventional quantization level of a unit macroblock is determined by the characteristic of the image signal in an image domain for processing image pixels. Thus, a signal processing procedure is complicated and hardware implimentation is difficult. Conventional technology using an energy characteristic of a spatial frequency domain to quantize the image signal is disclosed in U.S. Pat. No. 5,109,451.